A zinc electrowinning tankhouse uses cast lead-silver alloy anodes. Silver is added to lead anodes for electrowinning to reduce the rate of corrosion of the anodes in use. Lead anodes used in zinc electrowinning generally contain 0.5-1.0% silver. Lead-silver anodes used for zinc electrowinning contain no tin because tin will plate on the negative anode and prevent zinc deposits.
To produce good quality zinc the cathode in an electrowinning cell must contain less than 10 ppm lead. In order to reduce lead contamination of the cathode, the lead anode must be coated with a protective layer of PbO.sub.2 /MnO.sub.2. The silver present in the anode decreases the rate of initial oxidation of the anode surface leading to an extended time period before a stable oxide film is produced. Conditioning new anodes by developing a PbO.sub.2 /MnO.sub.2 layer on the surface normally takes many weeks. The complete formation of this layer may take as long as 60-90days. Until the anode is fully conditioned, the zinc cathodes in electrowinning cells experience high lead contents, high numbers of nodules and poor current efficiency. In addition, zinc production is substantially reduced as manganese ions are recirculated between anode and cathode as MnO.sub.2 spalled off the anode is reduced at the cathode to produce MnSO.sub.4. The production of zinc from a cell containing new unconditioned anodes may produce as much as one-third less zinc than corresponding conditioned cells.
Once a stable layer of PbO.sub.2 /MnO.sub.2 is formed on the anode, the current efficiency of the zinc electrowinning process increases dramatically, and the lead contamination of the resultant cathodes also decreases dramatically. Production of a stable PbO.sub.2, or PbO.sub.2 /MnO.sub.2 layer via pretreatment of the anode is described by Ecgett et al. in U.S. Pat. No. 3,880,733, Gaunce et al. in U.S. Pat. No. 3,392,094, Fountain et al. in U.S. Pat. No. 3,755,112, as well as R. H. Farmer in "Electrometallurgy" ed. H. Baker 1969. As described therein, a stable PbO.sub.2 layer/MnO.sub.2 layer is typically created by the immersion of the anodes in a preconditioning solution in which the anodes are electrolyzed to produce corroded layers. In some cases the anodes are first immersed in water or water and air to produce a PbO, Pb(OH).sub.2, or PbCO.sub.3 film which is more readily oxidized to a protective PbO.sub.2 layer than the normal cast or rolled surface. Rodrigues and Meyer, in "EPD Congress 1996" ed. G. Warren, describe the use of sandblasting to aid in preconditioning anodes.
Lead-silver alloy anodes are relatively weak. In use, they can become warped and bent leading to short circuits between the anode and cathode, low current efficiency, and lead contamination of the cathodes in the area of the short circuit. To improve the mechanical properties of the lead-silver anodes alloying elements such as calcium, strontium, barium and others have been added to the anodes to improve the mechanical properties. For example, UK patent application GB 2149424A by M. J. Thorn teaches an alloy containing 0.4-1.0% Ag, 0.05-0.15% Ca/Sr, less than 0.0002% antimony and optionally barium to reduce calcium losses during remelting.
Production of cast lead-silver or lead-silver-calcium anodes often results in the formation of numerous holes, voids or laps in the anode surface. In use, these can initiate internal corrosion in localized areas which can weaken the anode and cause warping. When the anodes are periodically cleaned of the adhering MnO.sub.2 deposit, the internal corrosion may cause cracking which can lead to premature anode failure.
To reduce the presence of internal porosity or laps, lead-silver or lead-calcium-silver alloys have been rolled into sheets. These sheets have been joined to a copper busbar by various means but primarily by welding the rolled sheet to lead which has been cast around the copper busbar. The rolled sheet generally has a smooth surface on which it is more difficult for the PbO.sub.2 /MnO.sub.2 corrosion product to produce an adherent film. In addition, the grail structure is uniform and is oriented in the rolling direction producing a grain structure with few grain boundaries available for corrosion and attachment of the oxidized film.
The improvement taught by this invention is the rolling of a cast billet of lead-silver alloys and treatment of the alloy during or after rolling at a temperature sufficiently high to produce a surface on which the PbO.sub.2 /MnO.sub.2 layer more readily adheres due to a grain structure having many grain boundaries. The grain structure is nonuniform (i.e., not oriented in the rolling direction). These anodes have more satisfactory mechanical characteristics than prior art cast anodes and can be conditioned much more rapidly than prior art rolled anodes.